The Securities and Exchange Commission’s potential plan to allow crypto companies to sell “tokenized” stocks could put investors at risk, a group of U.S. stock exchanges warned in a letter to the regulator this week.

Several crypto firms aim to offer tokens linked to listed equities, giving retail investors exposure to stocks without owning them directly. To legally sell these products in the U.S., crypto companies not registered as broker-dealers would require the SEC to issue a no-action letter or grant an exemption.